Container having a mixture of phase-change material and graphite powder

ABSTRACT

A container may be provided, in particular a beverage container, comprising an inner container wall and an outer container wall, an intermediate space lying between the container walls, wherein the intermediate space is filled with a mixture that contains phase-change material and graphite powder, wherein the graphite powder is introduced into the phase-change material in such a way that the mixture has a pulpy consistency at a temperature at which the phase-change material is liquid. A method may be provided for producing a container that includes the following steps: A) providing an inner container wall and an outer container wall; B) applying a mixture of graphite powder and phase-change material to the inside of the outer container wall and/or the outside of the inner container wall; C) joining the outer container wall and the inner container wall.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 nationalization of PCT/EP2014/078293, entitled “CONTAINER HAVING A MIXTURE OF PHASE-CHANGE MATERIAL AND GRAPHITE POWDER,” having an international filing date of Dec. 17, 2014, the entire contents of which are hereby incorporated by reference, which in turn claims priority under 35 USC §119 to German patent application 102013114507.8 filed on Dec. 19, 2013, entitled “Behälter mit einer Mischung aus Phasenwechselmaterial and Graphitpulver,” the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The application relates to a container having a mixture of phase-change material and graphite powder.

BACKGROUND

Document DE 10 2005 030 310 B3 discloses the placement of phase-change material between the inner wall and the outer wall of beverage containers. This is to ensure that a filled, hot beverage with a temperature above a desired drinking temperature releases heat to the phase-change material in that the phase-change material is melted. Thus a desired drinking temperature is attained. Further cooling due to loss of heat to the environment is reduced in that upon solidification of the phase-change material, heat is again released to the hot beverage. In order to introduce and then remove heat from the phase-change material efficiently, a heat-conducting structure is provided in the region between the inner wall and the outer wall. Beverage containers of this kind have positive properties for keeping hot beverages in the range of a desired drinking temperature over longer periods of time.

For the construction of building walls, ceiling and wall elements are known that contain a mixture of phase-change material and graphite. The phase-change material herein acts as a heat storage unit. The absorbed heat can be used to melt the solid phase-change material, such as wax. This occurs at the phase-change temperature. Thus a large amount of heat can be stored in a small temperature interval, namely in a temperature interval around the phase-change temperature. Upon solidification, the heat can be withdrawn again. These positive properties frequently cannot be used optimally, since the thermal conductivity of the phase-change materials is often low. Therefore, mixtures are used which contain graphite in addition to the phase-change material. In this way the heat conductivity can be improved significantly; in this respect see also the brochures “ECOPHIT: The Natural Graphite Building Material for Structural Air Conditioning,” “ECOPHIT G Highly Conducting Expanded Graphite Powder,” and “ECOPHIT S Flexible-Use PCM/Graphite Mixture.” Ceiling elements manufactured from such mixtures can be used, for example, to prevent overheating of a room. Excess heat causes the phase-change material to melt. Upon cooling of the room and the resultant solidification of the phase-change material, heat is released and counteracts the cooling. The heat conductivity is improved by the graphite powder.

Document DE 10 2005 051 570 A1 describes active features, such as heaters and cooling systems, supported by passive protective measures, for use in temperature stabilization in containers. To avoid maintenance and operating costs, containers have a double-wall design and the space between the walls is filled with a phase-change material, preferably paraffin or a paraffin mixture. The phase-change temperature is adjusted to a value within the permissible temperature range of the container interior space.

Document DE 10 2006 059 533 A1 discloses a thermal-regulated heat storage element. This element consists of a heat storage unit which uses different heat storage materials, preferably phase-change material, which is arranged on a thermal-regulating element and which ensures a temperature stability on its surface over a longer usage time without the addition of outside energy.

From U.S. Pat. No. 4,357,809A a beverage cooling arrangement with an inner receptacle is known. The inner receptacle has one closed end and one open end. The ends are connected by a wall. The outer encasement is provided for installation of a coolant.

Document U.S. Pat. No. 7,484,383 B1 discloses a pitcher which is supposed to keep beverages cool for a protracted period of time. The pitcher has a lining and a wall, between which a cavity is formed. A freezable substance is located therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a beverage container according to the invention; and

FIG. 2 shows a temperature profile of a filled, hot beverage container with and without a mixture containing phase-change material.

DETAILED DESCRIPTION

The object of the invention is to prepare an alternative beverage container which is easy to manufacture and to specify a corresponding manufacturing method. According to the invention, a container, in particular a beverage container, comprises an inner container wall and an outer container wall and an intermediate space lying between the container walls. The intermediate space is filled with a mixture which contains phase-change material and graphite powder, wherein the graphite powder is introduced into the phase-change material in such a way that the mixture has a sludgy consistency at a temperature at which the phase-change material is liquid.

This design provides several advantages. First, due to the mixture of graphite powder and phase-change material, sufficient heat conductivity into the phase-change material is possible. This effect is known from the ceiling and wall elements discussed above. Granted, this effect is also attained with the heat conducting structure known from the prior art.

One important, additional factor is that the mixture has a sludgy consistency at a temperature at which the phase-change material is liquid, as this means that leakage of the phase-change material is prevented. Although no phase-change material leaks from the sealed space between the inner wall and outer wall under normal circumstances anyway, such a mixture further inhibits such leakage. Even if the used phase-change material is generally not hazardous to health, so that any leakage—from damage to the container, for instance—would not be problematic, for reasons of food safety alone the appropriate protection is often desirable. But chiefly, any leakage of hot and liquid phase-change material would result in contamination. Also, leakage could potentially cause burns on the skin. It should be kept in mind, however, that the hot beverage itself can have a higher temperature. Although hot tea on the skin is less problematic than hot wax, for example, since the latter remains longer on the skin due to its higher viscosity. The heat released upon cooling and solidification of the wax—an otherwise usually desirable effect—would also be undesirable on the skin.

In addition, the mixture can be packed in a foil in order to reduce the potential leakage of the phase-change material. But from prior experience however, this does not appear to be necessary.

The mixture of graphite powder and phase-change material can also be readily adapted to any desired shape. Thus the specific shape between inner wall and outer wall of the beverage container can be easily attained. This is obviously the case at a temperature at which the phase-change material is liquid, that is, when the mixture has a sludgy consistency. But also at a lower temperature this is still usually quite possible. One need only keep in mind that at the right pressure, wax for example, can even be molded when it is largely solid. Thus it is possible to adapt to the specific shape of the beverage container at a reasonable expense. Hence economical manufacture is possible even in smaller quantities.

An additional factor is that due to its incorporation into the mixture with sludgy consistency, the liquid phase-change material cannot collect in the bottom region of the container, for example, meaning between the bottom of the inner wall and the outer wall. This problem is encountered with the containers known from the prior art. After filling of a hot beverage with a temperature markedly above the phase-change temperature, the phase-change material becomes liquid. Due to the force of gravity, it can collect in the bottom region and be absent everywhere else. Nor is this problem compensated for after subsequent solidification.

In one embodiment, the ratio of the useful container volume, that is, the volume available for the hot beverage, to the mixture volume amounts to 10:1 to 10:10, preferably 10:5 to 10:7. The larger the ratio, the smaller and lighter the container can be for a desired useful container volume. A larger quantity of the mixture is advantageous for rapid cooling to drinking temperature and a longer time at the drinking temperature. Thus a compromise has to be made. The designated ranges are possible. A ratio of 10:6 has proven to be expedient in practice.

In another embodiment, the phase-change material may be natural waxes, Montan waxes, fatty acids, in particular stearic acid, fatty acid esters, sugar alcohols, salt hydrates, paraffin, and/or stearic acid and/or waxes. The choice is governed primarily by the desired application. For example, stearic acid has a phase-change temperature of easily 60° C. and thus is within the range of desired drinking temperature for hot beverages. But salt hydrates usually have a lower transition temperature.

The melting range of stearic acid, depending on its composition, is between 60° C. and 70° C. In contrast to paraffin, which is a byproduct of petroleum processing, stearic acid is obtained primarily from plant-based palm oil or from animal fat. Palm oil is biologically decomposable and can be disposed of in an environmentally friendly manner, e.g. in biowaste.

In another embodiment of the invention, the phase-change temperature of the phase-change material is in the range of a desired consumption temperature. As was already discussed above, a desired drinking temperature for hot beverages is about 60° C. But baby food should be consumed at a temperature of about 30° C.

There is great potential in obtaining a suitable consumption temperature for baby food. For hygienic reasons, the use of boiling water is recommended for preparation of baby food. The resultant baby food should then be cooled down to consumption temperature and should not be additionally cooled thereafter. This cooling can be accelerated by the use of a container with phase-change material in the appropriate temperature range. The advantage here is evident, considering the impatience of babies waiting for the right consumption temperature. Maintaining a suitable consumption temperature for a longer period is also important.

Although from a current point of view the more important applications are to ensure a pleasant consumption temperature for warm beverages or foods, in principle it is also possible to quickly warm up cold beverages, for example, to a pleasant consumption temperature and to prevent additional heating by means of the phase-change material. In this case, of course, a much lower phase-change temperature would have to be selected.

In one embodiment of the invention, a structure is present in the intermediate space and contributes to the mechanical stability, wherein the structure can be a constituent of the inner and/or outer container wall. A structure of this kind can also be used—as described above in the prior art—to improve heat conduction and to prevent any flow of the liquid phase-change material. But generally the last-named effects are not necessary considering the good thermal conduction of the mixture—values up to 5 W/mK are attained—and the sludgy consistency of the mixture.

In another embodiment, the inner container wall has a thermal conductivity of at least 10 W/mK, especially preferably at least 20 W/mK, and/or the outer container wall has a coefficient of thermal conductivity of at least 0.01 m²K/W, preferably at least 0.1 m²K/W. The values for the inner container wall are attained by the use of metal, such as steel. Far greater values can also be obtained. A coefficient of thermal conductivity of 0.01 m²K/W can be attained by a normal plastic with a thermal conductivity of 0.2 W/mK and a thickness of 2 mm. A coefficient of thermal conductivity of 0.1 m²K/W can be achieved with an insulating material with a thickness of 4 mm and a thermal conductivity of 0.04 W/mK. As is known from the prior art, a high thermal conductivity of the inner container wall serves to conduct heat from the beverage or from the food to the phase-change material. A higher coefficient of thermal conductivity at the outer container wall helps to limit the dissipation of heat to the environment.

As was already touched on above, the invention also relates to a method for producing a container, in particular a container like one described above. The production process comprises the following steps, whose sequence of implementation is not initially specified:

A) providing an inner container wall and an outer container wall;

B) applying a mixture of graphite powder and phase-change material to the inside of the outer container wall and/or the outside of the inner container wall;

C) joining the outer container wall and the inner container wall.

Step A can be carried out in a manner long known in the prior art.

Regarding step B, it should be stated that there are several possibilities. One approach is to heat the mixture until the sludgy consistency is obtained. Then the mixture can be added to the inside of the outer container wall. Next, the inner container wall can be installed.

Alternatively, the inner container wall can first be introduced into the outer container wall. Next, the mixture can be introduced into the intermediate space, perhaps by means of an injection device. The mixture can be introduced into the upper region between the container walls, for example. It would also be possible to provide a sealable fill opening in one of the container walls.

Finally, the inner and outer container walls are joined according to step C. Here on the one hand, a certain amount of mechanical stability must be assured. But primarily a seal must be guaranteed, so that none of the mixture can leak from the intermediate space.

In particular, to date one proven method has been to mold the mixture into a shape which corresponds to the intermediate space between the inner container wall and the outer container wall. It is not necessary that the mixture have a temperature at which the phase-change material is in the liquid state. As already mentioned above, the mixture with solid phase-change material can be readily molded into the desired shape by sufficient pressure. The disadvantage that greater pressure is required here is made up for by the fact that the mixture in this state detaches again more readily from a molding tool. It should be mentioned that the corresponding molding tool can be removed comparatively easily. Thus even small quantities can be produced economically.

Even parts that are already on containers and are not designed according to the invention can be used as an inner container wall and outer container wall. Thus even containers produced in small quantities can profit from the advantages of large-batch production. It should be emphasized that this consideration is relative. Measured against the large number of containers, the number of containers according to the invention will always be small. But in absolute terms, this is an important innovation that may find broad application.

The invention will be described in greater detail below based on one exemplary embodiment. FIG. 1 shows a schematic of the beverage container according to the invention. FIG. 2 shows the temperature profile of a filled, hot beverage with and without a mixture containing phase-change material.

A beverage container 1 is shown in the form of a “coffee-to-go” mug. An outer container wall 2 made of plastic is depicted. A hot beverage can be poured into the useful container volume 3. The container volume 3 is enclosed by an inner container wall 4. The inner container wall 4 is formed by a metal insert with good thermal conductivity, which metal insert is made of aluminum or steel. There is an intermediate space 5 between the inner container wall 4 and the outer container wall 2. This space is filled with a mixture 6 which contains phase-change material and graphite powder. Even though FIG. 1 shows only a part of the intermediate space 5 filled with the mixture 6 for better visualization, it is basically reasonable to fill essentially the entire intermediate space 5 with the mixture 6. Of course, it should be noted that the mixture 6 has a somewhat smaller volume in the solid state of the phase-change material, than in the liquid state of the phase-change material. Thus it is clear that in the solid state of the phase-change material, the mixture 6 cannot fill the entire intermediate space 5.

To prevent heat losses, a layer of insulation (not shown) is applied to the side of the outer container wall 2 facing the intermediate space 5. Furthermore, a lid (not shown) is also supplied to cover the useful container volume 3.

Stearic acid is used as the phase-change material in the mixture 6. As the graphite powder, graphite-slurries are used, which are available from SGL-Carbon. The used stearic acid has a melting point, that is, a phase-change temperature, of about 69° C. The selected mixture 6 ensures that the mixture 6 is sludgy even at temperatures above 69° C., that is, when the stearic acid is melted.

For production of the mug 1, the mixture 6 with solidified phase-change material is molded with a molding tool into a fit mold. The fit mold is inserted in the outer container wall 2. The inner container wall 4 is inserted into the fit mold. The inner container wall 4 and the outer container wall 6 are welded together in the upper region.

FIG. 2 shows temperature profiles of a hot beverage poured into a mug. The temperature is plotted in ° C. on the vertical axis. The time is plotted in hours and minutes on the longitudinal axis. The temperature profile of a hot beverage is depicted over 1 hour and 30 minutes, wherein the hot beverage was 90° C. when added. The upper curve shows the temperature profile in a conventional “coffee to go” mug. The lower curve shows the temperature profile in one of the mugs 1 described above, with a mixture containing a phase-change material. The amount of the hot beverage in both cases was 200 ml. In the mug with the mixture 6 containing the phase-change material, the volume of the mixture 6 is about 120 ml. Both mugs have lids, which greatly retards cooling. At least in the embodiment with the lid, the advantage attainable with the phase-change material is clearly not long-term, slower cooling, as might have been supposed. Rather, rapid cooling to a pleasant drinking temperature is central, and after that only slow cooling. Thus the period of time in which a pleasant drinking temperature persists is greatly extended.

LIST OF REFERENCE NUMBERS

1 mug

2 outer container wall

3 useful container volume

4 inner container wall

5 intermediate space

6 mixture of phase-change material and graphite powder 

1. A container comprising an inner container wall and an outer container wall, wherein an intermediate space is between the container walls, wherein the intermediate space is filled with a mixture that contains phase-change material and graphite powder, wherein the graphite powder is introduced into the phase-change material in such a way that the mixture has a sludgy consistency at a temperature at which the phase-change material is liquid.
 2. The container according to claim 1, wherein the ratio of a useful container volume to mixture volume amounts to 10:1 to 10:10.
 3. The container according to claim 1, wherein the phase-change material comprises at least one of a natural wax, a Montan wax, a fatty acid, a fatty acid ester, a sugar alcohol, a salt hydrate, paraffin, stearic acid, or a wax.
 4. The container according to claim 1, wherein the phase-change temperature of the phase-change material lies in the range of the desired consumption temperature.
 5. The container according to claim 1, wherein a structure is present in the intermediate space which contributes to mechanical stability, wherein the structure can be a constituent of the inner and/or the outer container wall.
 6. The container according to claim 1, wherein the inner container wall has a thermal conductivity of at least 10 W/mK.
 7. A method for producing a container, the method comprising: A) providing an inner container wall and an outer container wall; B) applying a mixture of graphite powder and phase-change material to the inside of the outer container wall and/or the outside of the inner container wall; C) joining the outer container wall and the inner container wall.
 8. The method according to claim 7, wherein first step A) is carried out, then step B), and then step C).
 9. The method according to claim 7, wherein after step A), step C) is carried out before step B), and the applying the mixture in step B) comprises adding the mixture to an intermediate space between the inner container wall and the outer container wall, wherein the intermediate space was created in the joining in step C).
 10. The container according to claim 1, wherein the container is a beverage container.
 11. The container according to claim 2, wherein the ratio of a useful container volume to mixture volume amounts to 10:5 to 10:7.
 12. The container according to claim 3, wherein the phase-change material consists of stearic acid.
 13. The container according to claim 6, wherein the inner container wall has a thermal conductivity of at least 20 W/mK.
 14. The container according to claim 6, wherein the outer container wall has a thermal resistance of at least 0.01 m² K/W.
 15. The container according to claim 14, wherein the outer container wall has a thermal resistance of at least 0.1 m² K/W. 